Communication data traffic has been increasing in recent years. In order to hold such communication data traffic efficiently, high-capacity transmission is required also for long-haul optical transmission. It has recently been reported that there are limitations to transmission capacity per single mode fiber because the optical intensity that can be inputted into one core of a single mode fiber is limited due to non-linear effect in the fiber and the like. Accordingly, spatial multiplexing transmission technologies in which signals are multiplexed by using spatial degrees of freedom are being studied for further high-capacity transmission.
A multicore fiber having a plurality of cores in a clad of one fiber is proposed and developed as a transmission path to achieve the spatial multiplexing transmission, for example. In the spatial multiplexing transmission using a multicore fiber having N cores, it is possible to achieve a transmission capacity N times as large as a conventional single mode fiber having one core by transmitting different optical signals between cores.
It is also being studied to improve transmission characteristics by using the plurality of paths that are very close to each other. Non Patent Literature 1 discloses a method for improving signal characteristics due to the diversity effect by transmitting the same signals through a plurality of paths or the same signals subjected to different types of known scrambling for each path and combining received signals. According to the method disclosed in Non Patent Literature 1, the transmission capacity cannot be improved compared to a method using a single mode fiber having one core, but, by transmitting the same signals using N cores, transmission distances become approximately N times longer; consequently, long-haul transmission becomes possible.
The method described in Non Patent Literature is based on the following principle. That is to say, when the same signals divided into N pieces are combined in timing and in phase, their intensities become N2 times larger; in contrast, even when N pieces of uncorrelated noises are combined, their intensities become only N times larger; accordingly, an improvement effect of a signal-to-noise ratio can be obtained. The method is based on the above-described principle. However, if a timing gap even in the order of symbol time occurs among signals in combining the signals, no signal intensity increases, and it may rather cause the degradation. Consequently, in the transmission system in which the signals transmitted and received through the plurality of paths are made to cooperate, it becomes important to estimate delay difference between the signals and compensate for it.
For the long-haul spatial multiplexing transmission using a multicore fiber having N cores, a propagation delay difference between the cores is caused by manufacturing variations of each core, the difference in path length within an optical amplifier, a fan-out component, and each transceiver. The fan-out is an optical component that has the function of coupling optical signals to be inputted and output to a multicore fiber with a single mode fiber-based transceiver. The propagation delay difference is thought to be very small as compared to that in using N single mode fibers, but it is larger than a symbol time. As mentioned above, a timing gap arises when the timings of signals propagated through a plurality of paths are shifted at a receiving side due to a slight difference in path length and the like, and the amount of the timing gap is referred to as a delay difference between a plurality of signals in the following description.
The methods for estimating and compensating for the delay difference between a plurality of signals include a method of framing the signals and adding a specific overhead signal, as an example. In this method, a timing gap is obtained by detecting specific patterns from a plurality of received signals and comparing them. Then it is possible to align the plurality of signals based on the information. However, because timing alignment is performed in this method after completion of decoding each of the plurality of signals, it is necessary to perform the decoding process itself without using the information on the timing gap.
Consequently, it is difficult to apply this method to the method disclosed in Non Patent Literature NPL 1 in which a plurality of signals are combined and made to cooperate before the decoding. There is another method of repeating a process for decoding signals subjected to timing adjustment with a certain adjusted value, changing the adjusted value, until high-quality decoded signals are obtained. However, this method requires a large number of repeat counts if there is no information on an optimum value.
In contrast, as a method for estimating delay difference between a plurality of wavelengths without decoding each signal, there is a chromatic dispersion estimation technology. In the chromatic dispersion estimation technology, a chromatic dispersion is detected by detecting propagation delay difference between a plurality of optical signals that propagate through a fiber and slightly differ in wavelength from each other. An example of such a chromatic dispersion estimation technology is described in Patent Literature 1.
In the related chromatic dispersion value calculation method described in Patent Literature 1, as schematically illustrated in FIG. 12, a training signal in which the intensity is concentrated in two specific frequency components is periodically inserted into a signal to be transmitted. The training signal in which the intensity is concentrated in the two specific frequency components is referred to as a frequency signal. In the example illustrated in FIG. 12, a frequency signal with L symbols in length is inserted following a data signal with R symbols. The frequency signal has only frequency components of ±f0.
FIG. 13 illustrates a configuration of a chromatic dispersion calculating unit 100 included in a related optical signal receiving apparatus described in Patent Literature 1. All processes in the chromatic dispersion calculating unit are performed by digital signal processing. Received signals are branched into two signals; one of the signals passes through a band-pass filter (BPF) 111 with a passing frequency of +f0, and its intensity is calculated by an intensity calculating circuit 121. The other signal passes through a band-pass filter 112 with a passing frequency of −f0, and its intensity is calculated by an intensity calculating circuit 122. A delay time calculating circuit 130 compares timings at peaks of the intensities for the calculated two frequency components, for example, and calculates a propagation delay difference between the two frequency components. The propagation delay difference is determined by a frequency difference between the two frequency components and a chromatic dispersion amount accumulated in a transmission path.
Accordingly, a chromatic dispersion amount calculating circuit 140 calculates a chromatic dispersion amount from the calculated propagation delay difference and an already-known frequency difference.
As the related technologies, there are technologies described in Patent Literature 2 and Patent Literature 3